1. Field of the Invention
The invention relates to a control apparatus for an AC-AC direct converter in which a multiphase AC voltage is directly converted into a multiphase AC voltage of any desired amplitude and frequency by turning ON and OFF semiconductor switching elements.
2. Description of the Related Art
A matrix converter has been known as an AC-AC direct converter of the type specified above. Hereinbelow, the matrix converter will be taken as an example, and the configuration and operation of a control apparatus for the matrix converter will be explained.
First of all, FIG. 8 is a diagram showing the configuration of a main circuit of the matrix converter. Referring to FIG. 8, letters R, S and T denote AC input terminals that are connected to a three-phase power system or the like (also input phases of the power system shall be denoted by the same letters), and letters U, V and W denote AC output terminals to which a load is connected (also output phases of the matrix converter shall be denoted by the same letters). Bidirectional switches Sru, Ssu, Stu, Srv, Ssv, Stv, Srw, Ssw and Stw, each of which is capable of controlling a current bidirectionally, are connected between the input and output terminals. The matrix converter does not include an energy buffer of large capacity, and it directly derives an AC input voltage by ON/OFF controlling semiconductor switching elements which constitute the bidirectional switches Sru, Ssu, Stu, Srv, Ssv, Stv, Srw, Ssw and Stw, thereby to obtain an AC output voltage having any desired amplitude and frequency.
FIG. 9 is a block diagram of a control apparatus for the matrix converter. There are various conventional control schemes for the matrix converter. Among those conventional schemes, FIG. 9 shows an exemplary control scheme wherein the ON/OFF commands of the individual semiconductor switching elements constituting the matrix converter are created by supposing a system in which virtual rectifier 100 and virtual inverter 200 subjected to a PWM control are combined through virtual DC intermediate capacitor 300 as shown in FIG. 11. In FIG. 11, letters S1r-S6r denote semiconductor switching elements that constitute the virtual rectifier 100, while letters S1-S6 denote semiconductor switching elements that constitute the virtual inverter 200.
Referring to FIG. 9, virtual rectifier command calculator 21 calculates input current command values ir*, is* and it* of the virtual rectifier 100, and virtual inverter command calculator 23 calculates output voltage command values vu*, vv* and vw* of the virtual inverter 200. Both the input current command values and the output voltage command values are respectively calculated by the same methods as in a conventional rectifier and inverter. Switching pattern calculators 22 and 24 respectively calculate the switching patterns (ON/OFF commands) of the individual switching elements S1r-S6r and S1-S6 constituting the virtual rectifier 100 and the virtual inverter 200, on the basis of the input current command values ir*, is* and it* and the output voltage command values vu*, vv* and vw*.
FIG. 10 is a block diagram showing the configuration of the virtual inverter command calculator 23, in which a general V/f constant control is applied. Referring to FIG. 10, when a frequency command f* is inputted, a d-axis voltage command value vd* and a q-axis voltage command value vq* of a d-q rotating coordinate system are calculated and outputted by V/f constant controller 231. In rotating coordinate transformation block 232, the voltage command values vd* and vq* are subjected to rotating coordinate transformations by using a phase command θ* that has been obtained by integrating the frequency command f* by phase calculator 233, whereby the three-phase voltage commands vu*, vv* and vw* are outputted.
In command synthesizer 25 of FIG. 9, the switching patterns of the virtual rectifier 100 and virtual inverter 200 are synthesized. For example, a state where the switching element S1r within the virtual rectifier 100 and the switching element S2 within the virtual inverter 200 are ON in FIG. 11 is a state where the R-phase on the input side and the V-phase on the output side are connected. This state corresponds to a state in which the bidirectional switch Srv connected between the R- and V-phases is ON in the matrix converter shown in FIG. 8.
The calculations of the following Equation 1 are executed on the basis of such an idea, whereby the switching pattern of the bidirectional switches Sru, Ssu, Stu, Srv, Ssv, Stv, Srw, Ssw and Stw of the matrix converter can be uniquely obtained from the switching patterns of the respective switching elements S1r-S6r and S1-S6 of the virtual rectifier 100 and virtual inverter 200.
                              [                                                                      S                  ru                                                                              S                  su                                                                              S                  tu                                                                                                      S                  rv                                                                              S                  sv                                                                              S                  tv                                                                                                      S                  rw                                                                              S                  sw                                                            Stw                                              ]                =                                  ⁢                  [                                          ⁢                                                                                                                              ⁢                                                                                    S                                                                                                                                  ⁢                                                      1                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          1                                                                                      ⁢                                                                                  +                                                                                  ⁢                                                                  S                                                                                                                                  ⁢                                                      4                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          4                                                                                                                                                                                                                          ⁢                                                                                    S                                                                                                                                  ⁢                                                      2                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          1                                                                                      ⁢                                                                                  +                                                                                  ⁢                                                                  S                                                                                                                                  ⁢                                                      5                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          4                                                                                                                                                                                                                          ⁢                                                                                    S                                                                                                                                  ⁢                                                      3                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          1                                                                                      ⁢                                                                                  +                                                                                  ⁢                                                                  S                                                                                                                                  ⁢                                                      6                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          4                                                                                                                                                                                                                                                  ⁢                                                                                    S                                                                                                                                  ⁢                                                      1                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          2                                                                                      ⁢                                                                                  +                                                                                  ⁢                                                                  S                                                                                                                                  ⁢                                                      4                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          5                                                                                                                                                                                                                          ⁢                                                                                    S                                                                                                                                  ⁢                                                      2                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          2                                                                                      ⁢                                                                                  +                                                                                  ⁢                                                                  S                                                                                                                                  ⁢                                                      5                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          5                                                                                                                                                                                                                          ⁢                                                                                    S                                                                                                                                  ⁢                                                      3                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          2                                                                                      ⁢                                                                                  +                                                                                  ⁢                                                                  S                                                                                                                                  ⁢                                                      6                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          5                                                                                                                                                                                                                                                  ⁢                                                                                    S                                                                                                                                  ⁢                                                      1                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          3                                                                                      ⁢                                                                                  +                                                                                  ⁢                                                                  S                                                                                                                                  ⁢                                                      4                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          6                                                                                                                                                                                                                          ⁢                                                                                    S                                                                                                                                  ⁢                                                      2                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          3                                                                                      ⁢                                                                                  +                                                                                  ⁢                                                                  S                                                                                                                                  ⁢                                                      5                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          6                                                                                                                                                                                                                          ⁢                                                                                    S                                                                                                                                  ⁢                                                      3                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          3                                                                                      ⁢                                                                                  +                                                                                  ⁢                                                                  S                                                                                                                                  ⁢                                                      6                            ⁢                                                                                                                  ⁢                            r                                                                                              ⁢                                                                                          ⁢                                              S                                                                                                                                  ⁢                          6                                                                                                                                                  ]                                    [                  Equation          ⁢                                          ⁢          1                ]            
The terms Sru, Ssu, Stu, Srv, Ssv, Stv, Srw, Ssw and Stw, and S1r-S6r and S1-S6 in Equation 1 indicate the switching functions of the corresponding bidirectional switches and switching elements designated by the same letters, and they become “1's” when the bidirectional switches or switching elements are ON, whereas they become “0's” when OFF. This control scheme can apply directly to the controls of the conventional rectifier and inverter, and is therefore easy to realize. A technique of this type is disclosed by Jun-ichi Itoh et al., Proc. IEEJ-D Society, Vol. 124, No. 5, pp. 457-463, 2004.
Meanwhile, the matrix converter shown in FIG. 8 switches a three-phase AC voltage directly by the bidirectional switches, thereby to output a three-phase AC voltage. Therefore, the range of the voltage that can be outputted by the PWM control (PWM permissible range) lies within the envelope range of six-phase AC as shown in FIG. 12. Accordingly, the range of the output voltage commands with which a desired output voltage is obtainable is 0.866 times a supply voltage at the maximum, as shown as a meshed part in FIG. 12.
Within the range that does not exceed 0.866 times the supply voltage, a distortion of the output voltage does not appear, and a sinusoidal wave can be outputted. However, because such a control for generating a sinusoidal wave output is performed continuously, sometimes it is necessary to raise an output current in order to increase, for example, the capacity of the apparatus, especially in the case of employing a motor as the load. Therefore, it is sometimes required to heighten the utilization factor of the supply voltage and to generate a voltage output in an overmodulation region in which the voltage output exceeds 0.866 times the supply voltage. However, when the voltage outputted from the matrix converter exceeds 0.866 times the supply voltage, the distortion appears in the output voltage as indicated by a bold line in FIG. 12, on account of the restriction of the PWM permissible range. This distortion contains, not only components which are odd-numbered harmonics of an output frequency, but also low frequency components peculiar to the matrix converter that are determined by an input frequency and an output frequency.
When a motor is connected to the matrix converter as the load of the matrix converter, the distortion of the output voltage brings about a pulsation of a torque that causes noise or destroys the motor. In particular, the torque pulsation ascribable to a low frequency component below the input frequency exerts a serious influence.
To overcome the above drawback, a conventional control method permits the motor connected as the load to operate only when it does not give rise to a distortion in an output voltage. In other words, in the overmodulation region where the output voltage exceeds 0.866 times the supply voltage, a magnetic flux is weakened in accordance with the input voltage of a power converter, thereby to suppress the output voltage. This technique is disclosed by Ikuya Sato et al., IEEJ-Semiconductor Power Conversion Society, Paper SPC-04-75, 2004, and by Japanese Patent Laid-Open number 5-260762 (Paragraphs [0010], [0012], and [0014]-[0020], FIG. 1, etc.).
When the prior-art technique disclosed by Ikuya Sato et al. or Japanese Patent Laid-Open number 5-260762 is employed, the distortion of the output voltage of the power converter can be suppressed to control the waveform thereof into a sinusoidal shape. Therefore, it is possible to prevent the nonuniform rotation of the motor and the occurrence of the noise. However, the weakening of the magnetic flux increases a torque current, and the increased torque current increases the loss of the motor that causes an abnormal overheating. Accordingly, the motor requires a larger capacity leaving a safety margin, which raises the cost of a system.